The invention relates to a particle-optical apparatus which includes:
a particle source for producing a primary beam of electrically charged particles which travel along an optical axis of the apparatus,
a sample holder for a sample to be irradiated by means of the apparatus,
a focusing device for forming a focus of the primary beam in the vicinity of the sample holder,
at least two Wien filters which are arranged between the particle source and the focusing device.
The invention also relates to a method of separating electrically charged particles according to direction of movement in such a particle-optical apparatus as will be described in detail hereinafter.
A particle-optical apparatus of this kind is known from U.S. Pat. No. 5,422,486.
Devices in which a focusing primary electron beam is moved across the sample to be examined are known as scanning electron microscopes (SEM). In a SEM an area of a sample to be examined is scanned by means of a focused primary beam of electrically charged particles, generally being electrons which travel along an optical axis of the apparatus. The energy of the primary electron beam in the SEM can vary within a wide range in dependence on the application. This value may lie between limits of the order of magnitude of 1 kV and 30 kV, but higher or lower values are not precluded.
During irradiation of the sample to be examined, electrically charged particles (usually electrons) are released in the sample. These electrons may have an energy which varies from a very low value to a value which is of the same order of magnitude as the energy of the primary beam, for example from 10% to practically 100% thereof. The energy of the low-energy released electrons has a value of the order of magnitude of from 1 to 50 eV; in that case they are called secondary electrons. The energy of the high-energy released electrons has a value of the order of magnitude of from 50 eV up to the energy value of the primary beam; in that case they are called backscatter electrons. The range of the Auger electrons, whose energy values lie between approximately 50 eV and 5 keV, lies within and is superposed on the gradually varying energy spectrum of the backscatter electrons. In the energy spectrum of the backscatter electrons such Auger electrons cause a slight intensity variation in the relevant energy range; this variation may be considered as a ripple which is superposed on the gradually varying background of the backscatter electrons. The location and the magnitude of the maximum values in said ripple are characteristic of the material in the sample. In order to detect the energy variation of the Auger electrons, use can be made of special, known Auger detectors which are capable of separating the spectrum of the Auger electrons from the spectrum of the backscatter electrons.
The energy and/or the energy distribution of the released electrons provides information as regards the nature and composition of the sample. These electrons are released at the side of the sample at which the primary beam is incident, after which they travel back against the direction of incidence of the primary electrons. When one or more Wien filters are arranged in the path of the secondary electrons thus traveling back, these electrons then pass through these filters in a direction which opposes the direction of the primary beam; moreover, the energy of these released electrons will deviate from the energy of the primary beam. Consequently, the released electrons are deflected away from the optical axis, so away from the primary beam, so that further deflection means can conduct these electrons to a detector without noticeably influencing the primary beam. The further deflection means according to the cited U.S. patent document consist of an electron mirror which is arranged between the electron source and the focusing device and to the side of the optical axis. An energy selected image of the sample is thus realized in known manner. With a view to the quality of the imaging of the electron source to a scan spot, it is necessary that the Wien filters do not noticeably disturb the primary beam; it is notably necessary that the dispersion of these Wien filters does not enlarge the scan spot of the primary beam (in which the electrons inevitably exhibit a given energy spread) focused on the sample. In the cited patent document this is achieved by utilizing two Wien filters, the second Wien filter canceling the dispersion caused by the first Wien filter.
The known particle-optical apparatus is arranged to produce images by means of secondary electrons, i.e. electrons which are released from the sample and whose energy lies between approximately 1 eV and 50 eV. However, it is often also desirable to form an image by means of released electrons of a different energy, for example Auger electrons whose energy may be, for example a factor of one hundred higher than the energy of the secondary electrons. In that case the electric and magnetic fields of the Wien filters must become proportionally stronger; such greater strength is inevitably accompanied by imaging defects which would inadmissibly affect the spot size of the focus primary beam and hence degrade the resolution of the image.
It is an object of the invention to provide a particle-optical apparatus of the kind set forth in the introductory part of claim 1 in which Auger images of the sample can be formed without seriously degrading the spatial resolution by the selection of the Auger energy range. To this end, the particle-optical apparatus according to the invention is characterized in that for each Wien filter there are provided means for generating a quadrupole field, which means are situated at the area of the respective Wien filter in such a manner that at least a part of each of the quadrupole fields coincides with the field of the associated Wien filter.
A sharp spot of the primary beam can thus be obtained such that the desired resolution is maintained. Notably the astigmatism caused by the Wien filters is thus reduced. Because of the described situation of the quadrupole fields, the quadrupole strength can be varied without at the same time necessitating adjustment of the strength of one or both fields of the Wien filters.
It would in principle be feasible to attempt and realize the desired correction of the imaging defects of the Wien filters by means of the stigmators which are already included in an electron microscope in order to correct astigmatism of the electron lenses which, after all, also produce a type of quadrupole field. These stigmators, however, are designed to make small corrections so that they do not have a suitably defined pole surface. This means that the correct location relative to the optical axis of the relevant stigmator fields is not defined well enough, so that the corrective effect in combination with the Wien filters cannot be suitably predicted and, moreover, the image may wander far from the optical axis upon variation of the stigmator energizing (xe2x80x9cdrifting awayxe2x80x9d of the image). A more serious drawback, however, is that a difference in magnification occurs between the x-z plane and the y-z plane because the position of the stigmator along the optical axis deviates from that of the Wien filter; upon formation of the scan spot by the imaging of the electron source, this difference can readily lead to a ratio of 2:3 of the angle of aperture of the primary beam at the area of the sample. This means that, for example the image defect due to spherical aberration of the objective (known to be proportional to the third power of said angle of aperture) exhibits a ratio of 8:27 of said x-z planes and said y-z planes, so that the circular symmetry of the scan spot, and hence the resolution of the microscope, would be seriously disturbed.
Three Wien filters are arranged between the electron source and the focusing device in an embodiment of the particle-optical apparatus according to the invention. In this configuration the angular spread caused by the dispersion of the first Wien filter (viewed from the electron source) is canceled by the subsequent second Wien filter. Due to the distance between the first and the second filter, however, the electrons of different energy in the primary beam are then subject to a given lateral shift so that in given circumstances the desired image resolution can no longer be achieved. In cases where this is objectionable, a third Wien filter can be arranged behind the second Wien filter. The second Wien filter is then energized to a slightly higher degree, so that the electrons of different energy coincide again in the third Wien filter. Due to the dispersive effect of the third filter, these electrons are deflected through such different angles that they do not exhibit a resultant angular variation, and hence no resultant transverse shift, upon departure from the filter.
In a further embodiment of the particle-optical apparatus according to the invention, the means for generating a quadrupole field are formed by two additional electrons which are arranged parallel to the optical axis and extend perpendicularly to the electrodes for generating the electrical field in the Wien filter.
The two electrodes which extend perpendicularly to the electrostatic electrodes and parallel to the optical axis can be formed by the pole faces for generating the magnetic field, provided that these faces are electrically insulated from the remainder of the apparatus. It is alternatively possible to provide separate electrically conductive electrodes on these pole faces; these electrodes are in that case secured to the pole faces via an electrically insulating intermediate layer. The above steps can be readily implemented in this embodiment, for example by adjusting the power supply source (sources) of the relevant Wien filter in such a manner that, in addition to the supply of a pure voltage difference, relative to a common zero level, to the field electrodes (+V and xe2x88x92V relative to zero volts), the two additional electrodes are connected to a common voltage. As a result of this step, no additional physical means will be required so as to generate the desired quadrupole field and, moreover, the center of the quadrupole field will always coincide with the center of the Wien filter field. In these circumstances the correction of the lens defects due to the filter will be optimum.
Another embodiment of the particle-optical apparatus according to the invention is provided with deflection means for deflecting electrically charged particles from the sample further away from the optical axis, which deflection means are connected between the electron source and the focusing device and to the side of the optical axis, and are arranged to generate a substantially radial electrical deflection field. This step enables a comparatively simple construction of the deflection system; moreover, not only a cylinder-symmetrical deflection field but also a spherical-symmetrical deflection field is thus possible. This offers the advantage that use can be made of comparatively long deflection electrodes with a comparatively small spacing without the electron beam striking the plates. For a given deflection angle, a comparatively low voltage then suffices on said electrodes.
In a further embodiment of the particle-optical apparatus according to the invention the deflection means are also arranged to generate a magnetic deflection field and to adjust the magnetic deflection field and the electrical deflection field independently of one another. These steps yield a flexible instrument which has focusing properties in dependence on its setting. A point of the sample which emits Auger electrons can thus be imaged as well as possible on the entrance surface of the detector, so that the Auger detector can offer a favorable spectral resolution.
The invention also relates to a method for separating electrically charged particles in a particle-optical apparatus according to direction of travel, in which apparatus
there is produced a primary beam of electrically charged particles which travel along an optical axis of the apparatus,
the primary beam passes through a Wien filter which is arranged between the particle source and the focusing device,
the focusing device forms an end focus of the primary beam in the vicinity of a sample holder for a sample to be irradiated by means of the apparatus,
electrically charged particles are released from the sample in response to the incidence of the primary beam, which released particles travel in the direction opposing the direction of travel of the primary beam,
said released particles are deflected away from the primary beam in the Wien filter.
According to a further aspect of the invention, the desired formation of Auger images of the sample without seriously degrading the spatial resolution due to the choice of the Auger energy range can also be achieved by means of only one Wien filter; this benefits the simplicity of the particle-optical apparatus.
In order to achieve the foregoing object, said method according to the invention is characterized in that an intermediate focus of the primary beam is formed substantially at the center of the Wien filter, and that at the area of the Wien filter there is generated a quadrupole field, at least part of which coincides with the field of the Wien filter.
The effect of the Wien filter on the primary beam is that a ray in this primary beam which has an energy other than the nominal energy (i.e. the energy at which no resultant deflection is caused by the filter) is deflected by the filter. This deflected ray appears to emanate substantially from one point at the center of the filter, viewed from the exit of the filter, so that this point becomes a virtual object point for the imaging elements succeeding the filter, notably the objective which focuses the beam on the sample. When the intermediate focus of the primary beam is positioned in said virtual object point, the objective unites all rays emanating from said object point in the associated image point again, irrespective of the angle enclosed relative to the optical axis by the rays emanating from said object point. Any angular variation in said object point due to the energy dispersion of the filter, therefore, does not cause a variation in the image point, so a variation of the size of the electron spot of the primary beam formed on the sample. Due to the choice of the location of the intermediate focus, therefore, the electron spot constitutes a dispersion-free image of said intermediate focus. This choice of the location of the intermediate focus also offers the advantage that any second and third Wien filters can be dispensed with, because apparently no dispersion is caused in the first filter, so that no Wien filters will be required either for correction thereof.
Advantageous versions of this method are defined in the dependent Claims.